Applying alkaline peroxide chemicals in a mechanical pulping system (APMP) may be traced back as early as 1962. Since then, there have been a number of different process ideas developed to apply the chemicals before or during early stages of refiner pulping. In recent years, an extensive and systematic investigation has been reported on how different chemical treatments in refiner mechanical pulping affect pulp property development and the process consumption. For hardwoods, it was observed that alkaline peroxide pretreatment in general gives better optical properties, better bleachability and higher pulp yield at similar strength properties when compared to other conventional chemical pretreatment, such as alkaline sulfite and cold caustic soda processes. When compared to a peroxide post-bleaching process, applying alkaline peroxide before refining has a tendency to give a higher bulk at a given tensile strength for some hardwood species, such as North American aspen.
In a very broad sense, alkaline peroxide refiner mechanical pulping is a type of pulping process where hydrogen peroxide and alkali in various forms, together with various amounts of different peroxide stabilizers, are applied to the lignocellulosic materials before or during defiberization and fibrillation in a refiner. In the early stage of development of this type of pulping process, two basic concepts were tried. One was to apply alkaline peroxide treatment on chips, to allow the bleaching reactions to complete or to approach completion before refining. The other basic concept was to apply all the alkaline peroxide at the refiner, either with no pretreatment or with stabilizers or other alkaline pretreatment prior to the alkaline peroxide application at the refiner.
Conventionally the inclusion of chemicals such as silicates prior to the refiner leads to a situation where scale forms on the processing equipment. The refiner area itself also can suffer due to the formation of silicate precipitates, especially in processing softwoods, which can lead to a glassing of the refiner plates.
The application of chemicals at a point downstream of the refiner has also been proposed. However these proposals did not encompass the use of chemical pretreatment or conditioning of the chips. In addition such downstream chemical addition appeared incompatible with high pressure refining conditions.